Electrophotography forms the technical basis for various well known imaging processes, including photocopying and some forms of laser printing. The basic electrophotographic process involves placing a uniform electrostatic charge on a photoreceptor (also referred to as a photoconductor element), imagewise exposing the photoreceptor to activating electromagnetic radiation, also referred to herein as "light", thereby dissipating the charge in the exposed areas, developing the resulting electrostatic latent image with a toner, and transferring the toner image from the photoreceptor to a final substrate or receptor, such as paper, either by direct transfer or via an intermediate transfer material.
Typically, a positively charged toner is attracted to those areas of the photoconductor element which retain a negative charge after the imagewise exposure, thereby forming a toner image which corresponds to the electrostatic latent image. The toner need not be positively charged. Some toners are attracted to the areas of the photoconductor element where the charge has been dissipated. The toner may be either a powdered material comprising a blend of polymer and colored particulates, typically carbon, or a liquid material of finely divided solids dispersed in an insulating liquid frequently referred to as a carrier liquid. Liquid toners are often preferable because they are capable of giving higher resolution images and require lower energy for image fixing than do dry toner. Typically, the carrier liquid is a hydrocarbon that has a low dielectric constant (e.g., less than 3). Examples of such carrier liquids include NORPAR.TM. and ISOPAR.TM. solvents from Exxon Chemical Company.
Direct transfer uses an electrostatic assist to transfer the toner from the photoreceptor to the final substrate. See e.g. U.S. Pat. Nos. 4,849,784; 5,010,370; and 5,115,277. This method, therefore, requires high voltages and generates ozone due to the coronas producing the electrostatic assist. Ozone may lead to degradation of certain components inside the electrophotographic apparatus if allowed to accumulate. In addition, if the transfer charge is too high, toner may shift from an imaged area to a non-imaged area. Toner may also scatter when the substrate separates from the photoreceptor. The electrostatic assist renders the process sensitive to humidity particularly in dry toner systems. Finally, if a multicolor image is being formed, it may be difficult to keep the colors in registration.
For those reasons, a two step process may be used in which the image is first transferred to an intermediate transfer element, such as a roll, belt, blanket, etc. The first step may be referred to as T1. The image is then transferred to the final substrate. This transfer may be called T2. Either or both of T1 and T2 may be accomplished by electrostatic assist. Alternatively, T1 and/or T2 may occur using heat and pressure. If both transfers occur via heat and pressure, the surface energy of the intermediate transfer element must be intermediate to the surface energy of the surface of the photoreceptor (usually a release layer) and the surface energy of the toner. Unfortunately, image quality sometimes deteriorates when an intermediate transfer element is used. Hollow characters and halos are among the more common defects.
With liquid toners, the presence of the carrier liquid further complicates the transfer steps. While the presence of carrier liquid may facilitate electrostatic transfer, transfer of the carrier liquid to the final substrate is undesirable. Since the toner particles in liquid toned systems are smaller than in dry systems, Van der Waals forces make dry transfer difficult.
Past approaches for transferring liquid toned images with heat and pressure have concentrated on drying or partially drying the toned images before transfer to the intermediate transfer element (see e.g. U.S. Pat. No. 5,552,869 and U.S. Pat. No. 5,650,253, and using an intermediate transfer element that is highly resistant to the carrier liquid (see e.g. U.S. Pat. Nos. 3,811,765; 4,522,866; 5,047,808; 5,099,286; 5,119,140; 5,337,129; and 5,340,679). The use of a carrier liquid resistant material in the intermediate transfer element simplifies removal of the carrier liquid from the intermediate transfer element so that passage of the carrier liquid to the final substrate and out of the system is minimized. Unfortunately, these, methods do not eliminate all image defects from the final image. Particularly since it is difficult to ensure that all areas of an image (e.g. both isolated dots and large solids areas) are dried to the same extent. Therefore, a improved method of drying and/or transfer is still needed for liquid electrophotographic systems.